Conventionally, a light beam is adjusted to a desired photometry by forming a beam at source with the reflector, and by then "working" clearly predetermined portions of the beam by the use of prisms or ribs appropriately formed in homologous zones of the glass.
More particularly, though not exclusively, the reflectors of modern headlamps include mathematically defined optical surfaces which allow a light beam of the desired configuration and photometry to be obtained without the intervention of any occulting means. In cooperation with this type of surface, small and carefully predetermined deflections have to be produced by the closure glass of the headlamp in order to confer its final form on the light beam. Thus, with predetermined zones of the reflector, there are associated zones of the glass which are also carefully predetermined and which include elements, having a suitable configuration, for diverting or distributing the luminous flux.
In addition, a headlamp needs to have a facility for changing the direction of the light beam, in respect of its elevation and optionally also in respect of its azimuth without in any way reducing the integrity of the beam. In other words, it is necessary to ensure that as far as possible, during small pivotal movements imparted to the reflector in order to adjust the direction of light emission, the zones of the glass or screen in which light is diverted remain properly aligned with the portions of the beam that are emitted by the associated zones of the reflector, in the absence of which the beam will inevitably be altered.
The well-known optical solution to this problem consists in using an intermediate screen to effect the deviation of the light, the closure glass or lens then being itself, for example, essentially plain. In this case, the screen is mounted rigidly over the opening of the reflector, so that the homologous zones of these two members are always aligned with each other, and the adjustment of orientation is obtained by making them pivot together.
However, situations do exist in which the presence of an intermediate screen is not desirable. This may be for reasons of economy, or of styling, or even for photometric reasons, and this last case applies particularly when it is required that light losses, which may typically be of the order of 10-20%, in the region of the screen are to be avoided. In that case, the light deviating ribs and/or prisms are imposed on the closure glass itself, this being fixed, while the reflector is caused to pivot only for the purpose of adjusting the orientation. This however reintroduces the problem of misalignment referred to above.
Furthermore, this problem is the more considerable since modern headlamps, for styling and aerodynamic reasons, have their cover glass or lens arranged at the transition between the front of the vehicle and the front wing or wheel arch, and are thus steeply inclined with respect to a plane perpendicular to the optical axis: this angle may reach 45 degrees. As a result, at least some zones of the glass are substantially remote from the reflector, and from the axis or axes about which the latter is pivotable, and the adjustment of orientation then causes particularly significant misalignments in these zones, these misalignments being undesirable and being defined between the said zones and the portions of the beam which are to pass through them. The photometry of the beam, as it leaves the glass, is thus excessively changed.